I decided to use 3/8 inch tubing for my second loop (NB/SB/mosfets only) ... but the original 3/8 tygon I ordered was 5/8 OD ... so it was the same OD as my 7/16 tubing and wasn't "smaller." I then ordered some 3/8 ID tubing with 1/2 OD ... but it was too flexible and wouldn't "hold the loop" so I ended up with the 3/8 ID, 5/8 OD tubing ... works fine (stayed with 3/8 ID, since I'd already ordered the fittings).

( I realize this isn't all that relevant to the OP's goal with compression fittings, but thought I'd post it for others new to this whole thing ... make sure you check ID and OD on tubing )